The OPF 2.0 spec is very clear that font files must be listed in the <manifest/> for the book to be correct. (Epubcheck doesn't parse or check CSS and thus doesn't see the references to complain.) But you are correct -- there is no official assigned MIME type for TrueType or OpenType fonts. Most common seems to be 'application/x-font-opentype' and 'application/x-font-truetype', which is what calibre uses.